I believe this is quite a common postcard, although your example is quite nice and I like the fact that it's uprated. Currently there are a number of similar used cards (but not uprated) priced at €4 or so on
ebay, buy it now.
For these type of items, it can be quite hard to place a value on them since the price they sell for is heavily dependent on if there are enough interested bidders (assuming an eBay-style auction). If this card is listed on
ebay for €1, it could easily sell for that price or even remain unsold. If two people happen to be interested in it, the price could go up easily. Personally I would pay around €2 to €4 for the card.
It's interesting that you were able to track down the person who probably wrote the card. However, unfortunately Adolf Flockemann does not seem to have been a particularly famous person, so I don't see the card's value being increased due to its possible sender. Of course, it might have value to genealogists or descendants of the person if you're able to track them down (for example, I have a crumpled old postcard addressed to my great-great-grandfather, monetary value would be practically 0, but if I saw it on
ebay I would bid quite a lot since it has personal value to me).